The present invention relates to film transport systems for motion picture projectors and, more particularly, to a film projector movement that is capable of transporting film print having frames spanning a predetermined number of perforations per frame and then switching to another film format having frames spanning a different number of perforations per frame, without interrupting the operation of the projector during the switchover.
Conventional 35 mm theatrical motion picture projectors employ a motor driven sprocket wheel which pulls the film intermittently through the film gate at a standard rate of twenty-four frames per second. During the period of film movement, a rotating shutter driven by a constant speed motor blacks out the screen to prevent blurring. The viewing audience is unaware of these moments of darkness due to the phenomenon know as "persistence of vision." Film is supplied to and taken away from the film gate and intermittent sprocket by constant speed sprockets on either side. The periodic movement created at the film gate is smoothed out by film loops on either side of the intermittent sprocket, which are maintained by the constant speed sprockets.
Current theatrical projectors are almost exclusively of the mechanical type. Typically, a single synchronous motor drives a drive shaft bearing multiple drive gears, each of which drives either the shutter or one of the film sprockets at the appropriate speed. The intermittent sprocket is driven by a device called a Geneva mechanism, the purpose of which is to translate one full revolution of the drive shaft into a ninety degree rotation of the intermittent sprocket followed by a stationary period for projection. The ninety degree rotation of a sixteen-tooth sprocket results in a four-perforation frame change (i.e., one "pulldown"). The four-perforation frame standard was established in the late 1800's to accommodate a projected aspect ratio of 1.33:1 and has not changed since that time. Consequently, commercial 35 mm projectors are designed for four-perforation pulldown.
While virtually all theatrical 35 mm projectors are of the mechanical design, there are several specialty projectors on the market which feature electronic pulldown. These designs rely upon a high response servomotor instead of the Geneva device to advance and position the film in the film gate.
In the late 1950's, the "1.85" screen format was developed in order to provide the audience with a "semi-widescreen look" without having to resort to anamorphic camera and projection lenses, used for true widescreen presentation, which optically squeeze a 2.35:1 aspect ratio into a 1.33:1, four-perforation frame. Approximately eighty-five percent of the films currently in release use the 1.85:1 format. To achieve this projected aspect ratio, a mask is simply inserted into the aperture of the projection gate. This mask covers the top and bottom areas of the projection frame, thereby increasing the width to height ratio of the picture. Consequently, the exposed images in these masked areas are never seen.
In the accompanying drawings, FIG. 1 shows this quite clearly. The crosshatched area 64 represents the usable film area which is wasted in the 1.85:1 projection format with a frame height of four perforations. The area represented by the reference numeral 66 corresponds to the optical soundtrack. One solution to the problem of wasted film area is to change to an alternate frame height standard which provides the same projectable area as shown in FIG. 1, but without the wasted picture area at the top and bottom. One such alternate frame standard is the three-perforation frame shown in FIG. 2. By eliminating much of the area previously wasted by masking, the same projectable area can be fitted into three perforations of film instead of four. Consequently, elimination of this "wasted" area results in a reduction in release print footage, and therefore cost, by about 25%.
While the three-perforation format is a step in the right direction, it is not the ultimate in film conservation since there is still some wasted area at the top and bottom that must be masked during projection. FIG. 3 illustrates the ultimate frame height for the 1.85:1 format wherein there is virtually no wasted film area. The standard 1.85 format has an established picture width which is limited by the space reserved on the left side of the film for the optical sound track. This limiting frame width, together with the 1.85:1 aspect ratio, establishes the minimum frame height of 0.446 inches. When a few thousandths are added for space between frames, this height corresponds to precisely 2.5 perforations of film length. The 2.5-perforation pulldown format represents a saving of about 37.5% when compared to the standard four-perforation format and is currently being proposed as an alternate industry release print standard.
From the foregoing, it is apparent that there are several alternate frame formats which are highly desirable for various reasons but which suffer from the problem of incompatibility with existing projection systems. The successful introduction of alternate frame format films into theatrical exhibition will require that these facilities be equipped with projectors that are capable of operating in all formats. It is essential that these projection systems retain the capability to project four-perforation format films along with the alternate formats, since anamorphic widescreen presentation requires the full four-perforation frame. Additionally, there will always be some "classic" films and others (e.g., trailers and public service announcements) which remain in the original four-perforation format.
Several designs have been proposed that attempt to provide three/four-perforation format changeability to existing projectors. However, the fundamental problem with these designs is that they require manual changeover of each individual sprocket in the projector when changing the format in either direction. This makes these designs wholly impractical due to time and manpower constraints. The present invention introduces a fully automatic switchable-pulldown film projector movement. This movement will permit the same theatrical projector to exhibit various alternate format films in "back-to-back" fashion without undue attention from technicians and without hesitation in the presentation. In this manner, the present invention satisfies the problems with the prior designs and provides further related advantages.